The disclosed invention relates generally to an intermediate frequency (IF) receiver for frequency division multiplexed signals, and more particularly is directed to a digital IF receiver for frequency division multiplexed (FDM) signals such as frequency modulation (FM) radio broadcast signals.
Frequency division multiplexed (FDM) communications utilizes adjacent frequency bands or channels commonly characterized by respective carrier frequencies, such frequency bands being in a specified bandwidth which is typically wideband. A commonly known example of wideband FDM communications is the amplitude modulation (AM) radio broadcast band, which in the U.S. is fixed at 550 KHz to 1600 KHz with the channels spaced 10 KHz apart. Another commonly known example of wideband FDM communications is the FM radio broadcast band, which in the U.S. utilizes a 20 MHz bandwidth, from 88 MHz to 108 MHz.
Typically, a receiver for FDM communications includes an IF receiver which converts a selected channel of the received modulated radio frequency (RF) signal to a modulated signal having a lower carrier frequency called the IF carrier. The converted signal is provided to detection and decoding circuitry which provides appropriate output signals, such as the left and right audio outputs of an FM stereo tuner.
Typically, IF receivers are mostly analog, sometimes with some digital processing after the actual tuner function (i.e., after the isolation of the selected channel).
Important considerations with analog IF receivers include the necessity of precision circuit manufacturing techniques and the attendant non-automated manual adjustments. Noise is a significant undesired component and must always be carefully considered, from design to assembly. Distortion must be considered throughout the entire IF receiver circuitry. Undesired mixer products are present and may distort the channel of interest, and mixer local oscillator feedthrough is a problem. Many of the analog components are bulky and not amenable to integration, and moreover are subject to drift over time and with temperature which must be considered and reasonably compensated. Analog filters inherently have non-linear phase characteristics.